1. Field of the Invention
The invention relates generally to an improved method and apparatus for digital image processing. More particularly, the invention relates to a novel method and apparatus for removing or at least minimizing blocking artifacts perceptible in a block processed image.
2. Description of the Prior Art
A scene can be perceived as some visual reality that is distributed in space and/or time. Ordinarily, a scene is what the human visual system perceives as variations in light-dependent stimuli such as brightness, contrast, color and depth cues.
A scene can be captured by an electronic imaging device and represented as a multi-dimensional digitized image of picture elements, i.e. pixels. The image can be displayed in many different ways, e.g. a photograph, on a computer monitor, etc. The image is composed of various parts which represent scene characteristics. For instance, a color photograph of a scene is typically a collection of red, green, blue and luminance images of the same scene.
An image can be electronically processed by segmentation into M.times.N blocks of pixels, where M and N are preselected integers. This is done to provide compatibility of the block sizes with the processing limitations of commercially available chips. For instance, 8.times.8 blocks conform to international compression standards set by JPEG (Joint Photographic Experts Group) and MPEG (Motion Picture Experts Group).
Block processing is utilized by many known processing routines, such as the one taught in U.S. patent application Ser. No. 08/440,639 filed May 15, 1995 by the present inventors and others, incorporated herein by reference. There, a pyramid image representation of the image is segmented into M.times.N overlapped blocks, and subjected to a variant Wiener filter.
The above and other methods of digital image processing, which are based on block processing, sometimes exhibit visible blocking artifacts due to discontinuities in the block boundaries of a reproduced image. One type of image discontinuity comes from independent processing (e.g. filtering) of each block which causes images that can be visually unpleasant to human observers who tend to see the discontinuities as artificial tiling. The overall quality of the observed image drops dramatically.
The blocking artifacts problem can sometimes be adequately dealt with by overlapping adjacent pixel blocks in both the horizontal and vertical directions. For instance, FIG. 2 shows a portion of an image containing 144 pixels which is segmented into M.times.N blocks 200 and 210 where M=N=8. Notice that block 210 overlaps block 200 by 4 pixels in the horizontal direction, i.e. the horizontal overlapping coefficient k.sub.h =4. Of course, the size of the blocks and the amount of overlap between blocks can be selected to meet whatever design criteria is specified. In this case, filtering of each overlapped block yields a 4.times.4 section of filtered pixels for each block as shown by the crosshatched regions. The remaining 2 pixel wide perimeter of each 8.times.8 block is discarded. Further details concerning the overlapping of adjacent pixel blocks is disclosed in U.S. patent application Ser. No. 08/427,457 filed Apr. 24, 1995 by Wober & Reisch (see particularly FIG. 7 and the accompanying text on pages 32-33 which are incorporated herein by reference).
When each block is processed in a different manner, such as being filtered differently from an adjacent block, the discontinuities evident from independent processing sometimes cannot be overcome by any amount of overlapping. For instance in the variant Wiener filtering method mentioned above, each overlapped block at each pyramid level is independently filtered with one of many predetermined variant Wiener filters, which may result in the appearance of unacceptable blocking artifacts along the borders of adjacent blocks.
U.S. Pat. No. 5,454,051 issued Sept. 26, 1995 to Smith discloses a method of reducing blocking artifacts created by block transform compression algorithms by applying a variable lowpass filter (blur) operation on block boundaries that is based on the frequency coefficients of the transformed data. However, several limitations are evident in the Smith method. First, his method is applicable only to unoverlapped blocks. Second, his method processes each side of a block individually. Third, his corrections are determined only in the frequency domain. And fourth, his method uses a two point filter limited to blurring only boundary pixels.
Consequently, the primary object of the present invention is to overcome the above and other problems by providing an improved method and system for removing or at least minimizing blocking artifacts in an image subsequent to independent processing of each block. This and other objects of the invention will, in part, appear hereinafter and, in part, be obvious when the following detailed description is read in conjunction with the drawings.